instrument clusters in motor vehicles typically use pointers driven by motors such as air core gauges. It is also known to use stepper motors for that application. Stepper motors have the advantage of better accuracy, thinner package size, lower power consumption, and lower system cost. Both air core movements and conventional stepper motors require permanent magnets. The cost of permanent magnets adds to the system cost, and magnetic variance in permanent magnets due to temperature leads to gauge inaccuracy.
Accordingly, by eliminating permanent magnets from stepper motors, the cost can be still further reduced and the magnetic variance should be reduced as well. As a further benefit, such a stepper motor might by employed in applications other than gauges. The present invention avoids the use of permanent magnets by freshly inducing magnetic poles in a rotor as needed, the magnetic strength of each pole decaying soon after the pole has been used in conjunction with a varying field to advance the rotor. The prior art contains teachings of multiple horsepower motors using induced poles in a rotor. An example of an AC synchronous motor of that type is shown in the U.S. Pat. No. 4,227,136 to Roesel, Jr. which uses a special exciter coil to "write" poles in permanent magnet material on a cylindrical rotor surface and has separate field windings for producing a field to coact with the poles. That patent does not, however, teach the application of induced poles to stepper motors or to motors having fixed pole locations on the rotor. Further, it does not teach using the field coils to induce poles in the rotor.